Method of making gal vannealed ferrous metal of improved solderability



United States Patent 3,418,174 METHOD OF MAKING GALVANNEALED FER- ROUSMETAL OF IMPROVED SOLDERABILITY Paul R. Carter, Monroevilie Borough, andLawrence E.

Helwig, Hampton Township, Allegheny County, Pa., assignors to UnitedStates Steel Corporation, a corporation of Delaware No Drawing. FiledAug. 20, 1964, Ser. No. 390,998

2 Claims. (Cl. 1486.35)

ABSTRACT OF THE DISCLOSURE A process for producing galvannealed ferrousmetal of improved solderability is disclosed. A ferrous metal base isgalvanized with zinc which contains aluminum. The coated base isgalvannealed by heating to substantially completely alloy the coatingwith the ferrous metal. The galvannealed material is treated with steam,preferably under pressure, to improve the solderability thereof.

This invention relates to a method of improving the solderability ofgalvannealed ferrous metal. The terms ferrous and ferrous metal, as usedherein, refer to iron and iron alloys.

Galvannealed ferrous metal articles such as sheet or strip are producedby passing galvanized, i.e. zinc coated, ferrous metal through a heatingzone that maintains the coating molten until it alloys substantiallycompletely with the ferrous base. Galvannealed coatings possess asilvery matte finish of relatively low reflectivity and have good paintadherence properties. However, the solderability of galvannealed ferrousmetal is very poor and precludes the use of the material where goodsolderability is required. It is currently thought that aluminumcontained in the zinc used to coat the ferrous metal during galvanizingbefore annealing is responsible for the poor solderability ofgalvannealed articles. Aluminum oxides readily and the oxide film, whichmay be a mixture including aluminum oxide, appearing at the coatingsurface is nearly impervious to solder fluxes and makes solderingextremely difficult. As much as from about 0.007 to 0.25% aluminum maybe present in the zinc metal used for coating. Although aluminum-freegalvanized coatings could be used to make galvannealed articles ofimproved solderability, this approach is not commercially practicalbecause a small amount of aluminum is desirable in galvanized coatingsand the same production lines are used for galvannealing as are employedto produce galvanized material.

Several methods of removing the undesirable surface oxide fromgalvannealed metal surfaces have been proposed, but most areobjectionable because they either interfere with paint adhesion or aredifiicult to control. One such method involves treating the oxidesurface with mineral acids to dissolve the oxide; however, thistechnique tends to remove too much coating along with the oxide andfurther requires extensive rinsing. Alkali treatments also remove theoxide but present difiiculties in subsequent painting operations becauseeven small 3,418,174 Patented Dec. 24, 1968 Iceamounts of residualalkali on the surface would be likely to interfere with paint adhesion.

The present invention provides a method of producing galvannealedferrous metal with improved solderability from material galvanized withzinc which contains some aluminum. According to the invention,galvanized ferrous metal which has been coated with aluminum-containingzinc is galvannealed by heating to substantially completely alloy thecoating with the base, aluminum oxide being formed at the coatingsurface, and the galvannealed ferrous metal is contacted, e.g. bydipping, spraying, etc. with water at a temperature of at least 80 C.Still greater improvements in solderability can be obtained by usingboiling water or steam. With relatively short treating times of up toabout 10 minutes the degree of solderability improvement is proportionalto the length of time the galvannealed ferrous metal is subjected to thetreatment. Although the precise explanation of how the solderability isimproved is not fully understood, it is believed that the improvementresults from hydration of the oxide film.

solderability can be evaluated by means of a test that measures thesolder flow of a standard size solder pellet. As the solder spot ismelted on a hot plate, 61g. at 525 F., the pellet will either flow orbead depending upon the flow properties of the sheet surface. Good flowproperties will cause the melted solder pellet to spread over a largearea with the result that the thickness of the solder spot (measuredwith a micrometer) is reduced accordingly. Hence low solder spotthicknesses indicate good solderability.

The following examples illustrate various embodiments of the invention:

Example I Two different samples of galvannealed sheet produced by acontinuous strip operation were immersed for various times (0 to 20minutes) in water at 80 C., 90 C. and 100 C. At each temperature, thelonger the specimens were immersed the better were the solder-flowproperties, as the following Tables I and II indicate. The specimensalso gained weight in water which indicates that a chemical reaction hadoccurred. Boiling water (100 C.) was found to be much more effectivethan water at lower temperatures, where the improvement was sporadic. At100 C. maximum improvement resulted in about 10 minutes.

TABLE I.EFFECT OF PRETREATMENT IN HOT WATER gg s mDER-FLOW PROPERTIES OFGALVANNEALED Sample No. 1

Boiling Water, Hot Water, Time in Water, mm. 100 C. 80 C.

Wt. Gain, Solder Wt. Gain, Solder mgJdm/ Ht, mils mgJdm. Ht. mils 0 96 096 0. 04 71 0. 14 85 0. 18 0. 22 67 0.26 26 0. 32 70 0. 41 13 0.32 73 NTNT 0. 57 66 0. 60 15 0. 96 83 O. 11 NT NT 1. 42 13 NT NT 3. 10 NT NINoTE.-N1=No test conducted.

FLOW PROPERTIES OF GALVANNEALED SHE Sample No. 2

Boiling Water, Hot Water, Hot Water, Time in water, min. 100 C. 90 C. 80C.

Wt. Gain, Solder Wt. Gain, Solder Wt. Gain, Solder mgJdm. Ht., mgJdnLHt., mgJdm. Ht., mils mils mils 78 0 78 0 78 NT NT 0.41 56 0. 10 49 0.1056 0.35 46 0. 39 54 0. 23 43 NT NT 0. 38 37 0. 41 44 0. 51 46 0.89 41 0.69 36 NT NT 0.86 26 0. 46 29 0.89 38 1.05 28 0.69 26 NT NT 1. 31 20 0.8229 1. 53 29 1. 69 33 0. 9T 20 NT NT NT NT 1. 38 52 2. 16 38 2.84 23 2.24 15 3. 16 29 3. 41 18 NOTE.-NT=N0 test conducted.

Example II can be improved considerably with hot water, i.e., 80

Specimens of galvannealed sheets were placed in a pressure vessel andheld at various steam pressures (0 to 15 p.s.i.) for various lengths oftime (0 to 10 minutes). The improvement in solder-flow-test performance,as reported in Table III, was greater when the specimens were kept incontact with steam for longer periods, or when higher pressures wereused. Maximum improvement occurred in about 10 minutes, at 0 p.s.i.,minutes at 5 psi, 3 /2 minutes at psi, and 1 /2 minutes at psi. (Thelast figure is estimated, since all specimens treated at 15 psi. hadreached the maximum solder improvement in the time it took to close thepressure vessel, to bring it up to 15 p.s.i., release the pressure, andremove the specimens.) The higher the steam pressure and therefore thehotter the steam, the shorter was the time required to reach maximumimprovement in solderability.

TABLE III.EFFECT OF STEAM ON SOLDIER-FLOW PROPERTIES [Sample No. 2]

1 Total times include the time to reach the indicated pressure, the 5 5time at pressure, and the time to exhaust steam to open vessel.

3 Control, no pretreatment.

C. or higher and solderability can be improved still further bytreatments in boiling water. Shorter time periods can be used to obtainequivalent improvement when the galvannealed material is subjected tosteam under pressure.

While we have shown and described only a single embodiment of ourinvention, it is apparent that modifications may arise. Therefore, we donot wish to be limited to the disclosure set forth but only by the scopeof the appended claims.

We claim:

1. A process for producing galvannealed ferrous metal of improvedsolderability comprising galvanizing a ferrous metal base with metalliczinc which contains aluminum to provide a coating on the ferrous metal,galvannealing said zinc-coated base by heating to substantiallycompletely alloy said coating with said ferrous metal and contactingsaid galvannealed ferrous metal with steam to improve the solderabiltythereof.

2. A process according to claim 1 wherein said steam is under pressure.

References Cited UNITED STATES PATENTS 287,076 10/1883 Young 1l7l022,884,350 4/1959 Saubestre 148-614 3,177,088 4/1965 Sievert 1171 14 X3,247,026 4/ 1966 Switzer 1486.27

OTHER REFERENCES Galvanizing Techniques In the .U.S.A., O.E.E.C. Reportby a group of European experts, published by the Organization forEuropean Economic Cooperation, Paris, 1953, pp. 36 and 37 relied on.

ALFRED L. LEAVITT, Primary Examiner.

U.S. Cl. X.R.

